The present invention relates to telephone systems and, more specifically, to interface circuits for interfacing the subscriber line circuit with a switching office.
In telephone systems, the subscriber line circuit is connected with its end switching office by an interface circuit which performs a number of communication functions such as providing battery feed voltage at a selected loop current into the subscriber loop, overvoltage and overcurrent protection, implementing the ringing function, conversion from the two-wire loop to four-wire system, encoding and decoding of the analog voice signals, and supervisory control functions. Historically, these communication functions have been addressed by a variety of electrical and electromechanical circuits that typically included resistive, inductive, and capacitive components with electro-mechanical switches. In addition to a general requirement for reliable operation, the subscriber interface circuit must meet certain `survivability` requirements with regard to short-duration high-voltage spikes caused by lightning strikes between the subscriber station and the end office and overvoltage conditions caused by a 60 Hz power line (e.g., 120 or 240 VAC) circuit crossed to a telephone subscriber loop, the latter condition typically occurring as a result of a storm. In order to provide a measure of protection against these abnormal voltage situations, the subscriber loop is provided with voltage spike protectors of various types that shunt the subscriber loop and function to limit the effect of a lightning-induced voltage spike. Additionally, fusible components are connected in the subscriber loop and are designed to disconnect the circuit in an overcurrent situation typically caused by the presence of power line voltages on the subscriber loop. The use of fused circuits provides an assurance of overcurrent protection but presents a maintenance and service requirement in that a serviceman must replace the fusible components. In addition to these protective devices, the discrete electrical components historically used in subscriber circuits, including transformers, balance inductors, and the like, have an inherent immunity as a consequence of their current carrying capability and thermal capacity.
With the advent of digital switching systems, semiconductor devices are used to implement the traditional interface circuit functions. For example, monolithic integrated-circuit devices, as represented by the HC-550X-series subscriber line interface circuit manufactured by the Semiconductor Analog Products Division of the Harris Corporation of Melbourne, Fla., provide many of the above-described functions in a single integrated circuit. When combined with a digital coder/decoder and related interface components, a compact and efficient interface can be provided.
In general, semiconductor devices have a lower tolerance to overvoltage and overcurrent situations as compared to the discrete electrical devices historically used in interface circuits. While the semiconductor devices provide all the intended circuit functions, questions as to ultimate survivability have tended to limit use of semiconductor interface circuits to private-branch exchanges where the probability of lightning induced voltage spikes and power line cross-over is lower than in the traditional subscriber to end office connection. The questions as to the survivability of semiconductor subscriber interface circuits are of special concern in distributed end office systems in which a relatively small number of subscribers are connected to respective subscriber interface circuits located in the immediate area of the subscribers and which are connected by a carrier to the central office, in contrast to system organizations where all the subscribers are connected to the central office. In the former arrangement, an undue number of subscriber interface circuit failures requires a serviceman to travel to each location to effect repair while in the latter situation all repairs can be accomplished at one location.